This invention relates to lasers and, in particular, to a method and an apparatus for employing laser light to generate high precision through-cuts in a target material. The apparatus and method are particularly useful in singulating multi-element semiconductor substrates for the purpose of separating the semiconductor elements.
Laser devices have been used to cut various materials for some time. Disadvantages of such devices in the cutting process are known. For example, the cut quality (surface smoothness) has been less than desirable, and resolidified material has had a tendency to accumulate on the surface cut. The lack of available precision laser cutting methods has been particularly troublesome in the semiconductor packaging industry.
With the increased miniaturization of electronic devices, there has been widespread interest in semiconductor packages that occupy a minimum amount of space. Various methods have been developed that allow semiconductor dies to be encapsulated in packages that are no larger or only slightly larger than the periphery of the semiconductor chip. For example, U.S. Pat. Nos. 5,663,106 and 5,776,796 disclose methods of semiconductor die encapsulation that are used to produce what are known as chip-size or chip-scale semi-conductor packages. In these methods, a number of semiconductor dies are placed on a substrate and a flowable encapsulation material is deposited around the semiconductor dies. The encapsulation material is allowed to cure and the semiconductor dies are then separated (singulated) from each other to form a number of chip-size or chip-scale semi-conductor packages.
Various methods have been used to singulate, i.e. cut-out, the semiconductor devices such as die punching, sawing, rotary knife cutting, water jet cutting and laser cutting. While each of these methods may provide satisfactory results in certain situations, precisely uniform semiconductor device singulation has proven to be an elusive goal. The lack of precision singulation techniques has become particularly troublesome in that the electronic device industry has proposed new standards for the acceptable variation in the outline dimensions of chip-scale semiconductor packages.
There are several other chip singulation needs in the semiconductor industry. The most common are the separation of individual circuit elements that have been lithographically printed or otherwise generated on substrates such as silicon, gallium arsenide, several forms of ceramic, or silicon-on-insulator laminates where the insulator is glass, quartz, or sapphire. In this area, precision device singulation has also been difficult to attain.
In view of the foregoing, it is apparent that there is a continued need in the semiconductor industry for a chip-scale singulation method and apparatus that provides high quality, precision cuts at an economically viable cutting rate. Particularly, there has been continued interest in laser cutting techniques that may be used to precisely singulate semiconductor devices in conformity with increasingly tight industry standards for semiconductor device outlines.
Laser-based micromachining techniques have been applied in the semiconductor device industry in the past. For instance, U.S. Pat. No. 5,593,606 discloses a method and apparatus for forming circular shaped vias, both through and blind, in multi-layered semiconductor materials such as printed wiring board. The ""606 patent describes a method utilizing ultraviolet laser light to ablate layered materials of different chemical compositions using a circular laser spot. The ""606 patent states an advantage of its apparatus and method as elimination of a saturation limit on the power density ablation rate per pulse. The ""606 patent explains that the xe2x80x9csaturation depth of a cut per laser pulse is reached when an increase in power density of the beam pulse striking the target does not produce an appreciable increase of depth of cut into the target [and] [t]his is especially true for the excimer laser because the use of a beam-shape controlling mask dictates that the beam spot area equal the spatial area defined by the via to be cut.xe2x80x9d The ""606 patent also notes that it is believed the saturation depth phenomenon is caused by xe2x80x9cdevelopment by the first beam pulse of a debris plume that acts as a filter or mask for subsequent beam pulses.xe2x80x9d
This known apparatus and method suffers a number of disadvantages. For example, by using a circular spot, cut roughness can increase. Another disadvantage is a saturation limit on the energy density ablation rate per pulse. If the energy of each individual pulse applied to the intended ablation volume (defined by the spot area and absorption depth) is less than the energy needed to vaporize the material, little or no ablation will occur. On the other hand, if the pulse energy is substantially greater than the energy needed by the ablation volume for vaporization, the excess energy is dissipated as heat or is otherwise not used for material removal. Either way, ablation efficiency is relatively low.
It has been discovered in the present invention that this effect can be minimized by using laser pulses with energy approximately matching the energy requirement for complete vaporization of the intended ablation volume. For a given material and laser wavelength, there is an optimum energy density whereby the ablation efficiency is maximized. Since the spot size in the method and apparatus of U.S. Pat. No. 5,593,606 is stated to be less than about 50 micrometers in diameter, optimum ablation efficiency would be achieved only if the pulse energy was very low. Since cutting rate is generally proportional to average power of the laser, high throughput would require a large laser repetition rate.
Another limitation on laser ablation is a requirement on the minimum intensity of the laser pulse. Although the pulse may have sufficient or optimum energy content, it can not be efficiently used for vaporization if the time of application (pulse width) is too long, that is, long compared to the characteristic thermal diffusion time of the material. Generally, the pulse width must be short enough such that thermal diffusion into the material is minimal.
Accordingly, there is a need for a laser cutting system and method that provides laser pulses of optimum energy and duration such that efficient ablation can occur resulting in effective cuts. The average power requirement and hence, cost, of such a device would be minimized.
The present invention alleviates to a great extent the disadvantages of the known apparatus and methods for forming through cuts in target materials by providing, in one embodiment, a laser system that amplifies and increases (such as tripling) the frequency of the output of a seed laser subsystem with one or more stages of amplification to generate ultraviolet light output pulses. These pulses are focused into an elongated orientation, such as preferably a line or ellipse with an optical focusing apparatus such as a cylindrical lens pair. The elongated, elliptical or line focus is projected onto the surface of the target material to be cut. The target material is moved in the same direction as the elongated focus, such as substantially parallel to the line focus in the preferred embodiment, to create a continuous through-cut. The lenses can be rotated and the material motion direction can be varied to allow cutting along any direction parallel to the plane of the target material.
The laser beam is focussed to an illumination area (also referred hereafter as a xe2x80x9cspotxe2x80x9d) shaped and dimensioned to include a major axis and a minor axis and preferably, the laser pulses are applied to the material such that the major axis of the illumination area moves or is moved parallel to the cutting direction. The illumination area also has a leading edge and a trailing edge on the major axis, and the energy density of each laser pulse increases from zero to a maximum along the leading edge and decreases back to zero along the trailing edge. The pulse energy of the laser pulses is selected to exceed the ablation energy density required for the material to be cut. In other words, the pulse energy selected is based on the ablation threshold energy of the target material and the area of the illumination area. In a preferred embodiment, the pulse energy is greater than about 5 microJoules, preferably is between about 0.1 milliJoules and about 250 milliJoules, although it should be understood that other energy levels can be selected.
When this method and apparatus are used in semiconductor packaging, the method and apparatus allow for the complete singulation of individual semiconductor elements, which often are rectangular (or square) and need to be cut on all four sides, although it should be recognized other shape cuts can be generated. In the preferred embodiment, the method and apparatus produces singulated semiconductor devices having tight dimensional control on the device outline as required by electronic device industry standards.
In the method of the present invention, the target material is cut. The steps of making a cut include generating laser pulses from a laser system and applying the laser pulses in a cutting direction to the target material so that the laser pulses cut through the target material. The laser pulses preferably have a line focus or an approximately ellipse shaped spot, have a pulse duration sufficiently short to minimize thermal diffusion, have an energy density sufficiently above the ablation threshold of the material being cut to ablate efficiently, and a repetition rate sufficient to achieve a desired cutting rate. In a preferred embodiment, the temporal pulse width is shorter than about 100 nanoseconds, and the energy density is between about 2 to about 20 times the ablation threshold energy of the target material. Any suitable repetition rate may be selected to achieve the desired cut rate and depth. In a preferred embodiment, the laser pulses have a repetition rate greater than about 50 Hz, and most preferably have a repetition rate between about 100 and about 1500 Hz.
In one embodiment of the invention, the laser system includes: a seed laser system for producing a first pulse laser beam; a laser amplifier for amplifying the first pulse laser beam to produce an amplified pulse laser beam; a harmonic generation system for converting the amplified pulse laser beam into a second pulse laser beam having a shorter wavelength; a focussing system for focussing the second pulse laser beam to an approximately ellipse shaped third pulse laser beam; and a motion control system for moving a target material contacted by the third pulse laser beam. Preferably, the focussing system includes two lenses. The first lens controls the length of the major axis of the approximately ellipse shaped third pulse laser beam and the second lens controls the length of the minor axis of the approximately ellipse shaped third pulse laser beam. The motion control system includes two orthogonal stages capable of moving the target material in any direction parallel to a surface plane of the target material.
One advantage of the present invention is that a cost-effective laser system and method is provided to form high quality through cuts in target materials, especially target materials used by the semiconductor industry.
Another advantage of the invention is that a laser cutting system and method that optimizes the energy density on the material and have a sufficiently short pulse duration such that ablation efficiency is maximized is provided.
The method and apparatus of the present invention also has other advantages, which should be apparent to one skilled in the art. By way of example, the proper selection of the width or, preferably, the length of the line focus can optimize the energy density. This allows the use of higher pulse energy lasers, which would be inefficient if the maximum spot dimension was constrained to a 50 micrometer diameter or less. Another advantage of the present invention is that for a specific average power, lower pulse repetition rate can be used because of the utilization of higher pulse energy.
Another advantage of the present invention is that the probability of shock damage is reduced because of operation at low energy density and because of energy density tapering at the leading and trailing edges of the cutting laser spot.
A further advantage is that by moving the line focus along its length, cuts are very straight with little width variation. A fifth advantage is that by using a high beam quality, high brightness laser, the volume of the cut and the amount of debris generated from the ablation is minimized.
These and other features and advantages of the present invention will be appreciated from a review of the following detailed description of the invention, along with the accompanying figures in which like reference numerals refer to like parts throughout.